The present study will examine the effects of smoking during pregnancy on the physical, behavioral and mental development of children born to mothers who smoked during pregnancy and who participated in a randomized clinical trial. Half of the mothers received an active intervention program to encourage and assist them in smoking cessation during pregnancy. These mothers had significantly lower salivary thiocyanate (a measurement of smoking) at the end of pregnancy relative to the control mothers. Comparisons in the proposed 2-year-old follow-up will be between these same randomly allocated groups: the group of children whose mothers had intervention and the group whose mothers did not. This will determine whether reduction of smoking in pregnancy has a long-term benefit for the child. The null hypothesis is that reduction of maternal smoking during pregnancy has no effect on children during the first two years of life as measured by: (a) physical growth (N = 1,000), (b) reported morbidity (N = 1,000), (c) hospital admissions (N = 200), (d) recording of EEG (N = 150), (e) recordings of event-related potentials (N = 150), (f) Bayley Infant Scales (N = 150), (g) Minnesota Developmental Scale (reported by parent) (N = 1,000). The follow-up cohort will consist of about 1,000 children, 150 of whome will get intensive neurometric and behavioral assessments.